Visualization of the High Surface-to-Volume Ratio of Nanomaterials and Its Consequences

When bulk materials are reduced in size to the nanometer scale, in particular, their surface-to-volume ratio increases drastically. We introduce some simple experiments on how to visualize this concept to students in the framework of a laboratory class. In the same context, experiments to demonstrate the consequences of this on the properties of the materials are introduced. This will involve solubility and chemical surface reactivity of the materials and properties originated from the surface. In the framework of their chemical reactivity, potential benefits and threads of nanomaterials due to their high surface-to-volume ratio will be discussed, such as applications as catalysts and their impact on nanotoxicology.

assessment: did the pupils understand that by cutting down materials the volume stays the same while the surface increases?
• perform variant i, variant iii, and variant iv; upon availability of time the cubes can already be preprepared by the teacher; limit the time for the gluing together of the cubes at any rate • perform the calculations • assessment: by cutting down a cube into 9 similar cubes, how much does the surface-to-volume-ratio increase?
• show variant i • alternatively you can take an object from daily life, such as an avocado: cut it with a knife into small pieces and explain that while the mass of the avocado remained constant, the pieces possess a higher surface • skip the calculations • assessment: ask the question whether the audience understood that cutting down a material into smaller pieces increases the surface-tovolume-ratio Influence of the surfaceto-volume ratio on the dissolution and melting properties of materials

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perform the melting of the ice cubes experiment • assessment: asked for the reason why the smaller cubes melt faster the pupils should be able to answer that it is due to the higher surface-tovolume ratio • perform the melting of the ice cubes experiment • perform the rock sugar dissolution experiment; optionally also perform the experiment with the sugar loaf and the the pupils discuss why this experiment fails • assessment: asked for the reason why the smaller cubes melt faster the pupils should be able to answer that it is due to the higher surface-to-volume ratio • perform the melting of the ice cubes experiment in a simplified way: take 2 ice cubes.One is crashed by a hammer in small pieces.Put the intact ice cube in one glass nd the smashed pieces in another glass.At the end of the lecture the small pieces should have molten to water, whereas the intact ice cube is not fully molten • assessment: asked the question whether the audience understood that the smaller cubes melt faster due to the higher surface-to-volume ratio Influence of the surfaceto-volume ratio on the catalytic properties of materials • the teacher should either perform the colorimetric assay by him/herself.Alternatively, the pupils may also do the pipetting of the preprepared solutions.Especial care has to be taken about wearing googles, gloves, and coated • the reaction equation does not need to be show, it is only important that the pupils see whether a reaction happens or not • assessment: this part is complicated for the pupils, as they have not learned in school what "catalysis" is.Discuss this based on the Supporting Figure S5 and ask by questioning if the general concept is understood

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The pupils should do the pipetting of the preprepared solutions.Especial care has to be taken about wearing googles, gloves, and coated

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The reaction equation should be discussed • assessment: discuss with the students about the potential use of nanomaterials for catalytic applications.Ask how a catalytic converter in a car could be improved (give as a hint that it depends on the available surface of Pt).Ask why nanomaterials thus could also impose risks.The pupils should be able to tell that the high surface-to-volume ratio may make materials catalytically active and thus reactive • discuss surface reactivity at the example of a cut avocado: after a while, the surface gets brownish.While a freshly peeled avocado will turn brown only at its outer surface, an avocado cut into small pieces will be fully turn brown • discuss the perspectives for using nanomaterials for applications and the potential risks: mention the catalytic converter of cars, which involves Pt.Pt nanostructured Pt has a higher surface-to-volume ratio and thus performs better.Consequently, to achieve the same performance less nano-particulate Pt is required than bulk Pt and thus less weight.Mention then that also surface which are barely reactive with bulk materials (golden ring), in case of nano-particulate form (gold nanoparticles) they may have measurable surface reactivity (catalytic effects) • assessment: ask for the benefits and risks of nanomaterials.This generally will lead to a discussion about risk-assessment and variant iii; the pupils should actually glue the paper cubes together • skip the calculations •